During manufacturing of semiconductor devices, such as power devices, doping regions are often formed by implantation. The final doping profile, that is, the spatial variation of the doping concentration in the semiconductor material, of the respective doping regions is influenced by many factors such as implantation dose, implantation energy and thermal processes which the semiconductor device is subjected to during the manufacturing process. For instance, thermal processes cause diffusion of the dopant in the semiconductor material which results in a broadening of the doping regions. On the other hand, thermal processes are required to activate the dopant and to remove crystal damages of the semiconductor material caused by implantation.
During thermal processing, the shape and extension of the doping regions, which are defined by implantation, change. As a consequence, sharp doping transitions are partially softened. Doping regions formed at an early stage of the manufacturing process are subjected to more thermal processing than doping regions formed at a later stage. The “resolution” of the “early” doping regions in lateral and vertical direction is therefore more deteriorated than that of the “late” doping regions.
To reduce the impact of thermal processes attempts have been made to rearrange the sequence of the individual processing steps such that the thermal processes having a high thermal budget are carried out at the beginning of the manufacturing process while processing steps for forming doping regions having sharp transitions are shifted to the end of the manufacturing process. Such re-arrangement of the processing steps is often not possible, such as when the formation of doping areas arranged in the deep volume of the semiconductor material are concerned.
Doping regions arranged in the deep volume of the semiconductor material can be formed by deep implantation. Deep implantation, however, requires large implantation energies and thick implantation masks to reliably cover those portions of the semiconductor material in which no implantation is desired. Moreover, lateral and vertical straggling may cause broadening of the doping profile. Alternatively, the doping regions can be formed by shallow implantation followed by epitaxial formation of a semiconductor layer to bury the doping regions. Since epitaxial formation is typically formed at the beginning of the manufacturing process the thus formed doping regions is subjected to most of the thermal processes.